There has been known a control apparatus which controls an inverter to feed a coil current to a rotary magnet type AC motor in which a permanent magnet is provided in a rotor and a coil is provided in a stator. If an inverter is used, a frequency and a conduction phase of the coil current can be controlled, and a control scheme including a pulse width modulation scheme has been put to practical use. In addition, in order to secure high controllability, not only is an electric input is controlled according to a necessary required output, but a rotational position of the rotor and the coil current are also sequentially detected so as to perform a feed-back control, and a biaxial theory to be described later also belongs thereto.
An example of such a control apparatus is disclosed in Patent Document 1. The control apparatus of a motor driving system in Patent Document 1 is provided with a first and second motor control mechanism, and a control mode selecting mechanism selectively sets a control mode according to a driving state of the AC motor. As for the control mode, a PWM (pulse width modulation) control mode and a rectangular wave power control mode are provided, and the former mode is subdivided into a sinusoidal wave PWM and an overmodulation PWM. According to the Patent Document 1, the control apparatus can dispense with a conventionally-required voltage sensor for detecting a voltage of a DC voltage source on the input side of the inverter, and can eliminate a torque difference caused by detection error of the voltage sensor when the control mode is switched.
In addition, an electronic control apparatus is used in the control apparatus, and calculation is generally performed according to the control mode. The biaxial theory is known as a theory of controlling a three-phase rotary magnet type AC motor and is described in Non-Patent Document 1 and the like. In the biaxial theory, the direction of the N pole of the permanent magnet on the rotor is assumed as the d axis, the direction rotating by the electric angle 90° from the d axis is assumed as the q axis, a coil current vector of three phases is converted into a DC current of two axes on the dq coordinate axis based on the rotational position of the rotor, an amount of change to be controlled is obtained by performing calculation on the dq coordinate axis and then performs an inverse conversion so as to obtain an amount of change in the three phase area. By the biaxial theory, various control schemes are established for controlling a current vector of three phases.    Patent Document 1: JP-A-2007-159368    Non-Patent Document 1: Interior Permanent Magnet Synchronous Motor by Takeda, Yoji and co-writers, Obunsha, published in October 2001
When the inverter is controlled in the pulse width modulation scheme on the basis of the amount of change obtained by the biaxial theory so as to drive the AC motor, the control apparatus needs to switch the pulse width modulation scheme from the sinusoidal wave control mode to the overmodulation control mode as the required output is increased. In the sinusoidal wave control mode, since a reference triangle waveform is simply subtracted from the sinusoidal waveform corresponding to the required output so as to generate a pulse waveform representing an energizing phase, a calculation load of the control apparatus can be reduced, and the control can be performed in a short cycle time. In addition, in the overmodulation control mode, since a peak of the sinusoidal waveform exceeds the reference triangle waveform, calculation for correcting the sinusoidal waveform is necessary in addition to the subtraction, the calculation load of the control apparatus is increased, and the control requires a long cycle time to be performed. In other words, in the sinusoidal wave control mode and the overmodulation control mode, the cycle times for performing the control are different from each other.
On the other hand, in the calculation for obtaining the amount of change to be controlled on the dq coordinate axis by the biaxial theory, a constant parameter is used by a proportional-integral-derivative control (PID) or the like in many cases. The parameter may be properly set according to a period of the cycle time. If a common parameter is used in the sinusoidal wave control mode and the overmodulation control mode, the control is performed excessively so that the stability is degraded in which the output of the motor oscillates to be large or small, or too little control is performed so that responsiveness is degraded in which tracing to the load change is delayed.
In addition, when the parameters are separately used in the sinusoidal wave control mode and the overmodulation control mode, a proper control is performed in each mode, but the amount of change obtained before and after the mode switching is inconsistent, and the output may be discontinuous. If the cycle time in the sinusoidal wave control mode is made long to match with the cycle time in the overmodulation control mode, and further the common parameter is used, the inconsistency in the mode switching can be solved, but the rapid responsiveness in the sinusoidal wave control mode is sacrificed.
Thus, a need exist for a control apparatus and a control method of an AC motor, which is not susceptible to the drawback mentioned above.